Various tools have been developed for downhole cutting or severing of casing strings in wellbores, and for cutting or milling window sections in casing strings. Generally, such tools have comprised a main body with multiple hinged arms or blades, which are rotated outwardly into contact with the casing (by hydraulic or other means) when the tool is in position downhole. Usually, fluid is pumped down through the drillstring and through the tool to actuate the mechanism and rotate the blades outward. Once the blades are rotated outwardly, rotation of the drillstring (and tool) causes the cutting surfaces on the blades to cut through the casing string. Fluids are pumped through the system to lift the cuttings to the surface. Known tools, however, cannot efficiently cut or sever multiple, cemented-together casing strings, and in particular cannot efficiently cut “windows” in such strings; by the term “window” is meant the cutting or milling of a section (e.g. 20′) of the casing string, as opposed to simply severing same. In addition, known tools tend to form long, connected metal shavings which must be lifted from the wellbore by the fluid flow, else same become nested together downhole and potentially cause the drillstring to become stuck.